1. Field of the Invention
The present invention relates to a method of and an apparatus for generating a tracking error signal used in a servo tracking control for controlling the irradiation position of the light beam for recording and reproducing information, to track or follow up the information track on an information recording medium.
2. Description of the Related Art
When optically reproducing the information, which has been recorded on the information recording medium such as an optical disk, it is necessary to focus a light beam for reproducing the information at the optimum focal length, with respect to the information track, which is formed on the optical disk or the like and on which the information is recorded, and to control the irradiation position of the light beam (i.e. the light spot) to track or follow up the information track.
At this time, in an optical disk reproducing apparatus, in order to focus the light beam onto the information track, the focus servo control is employed by using a focus error signal produced by the reflected light from the optical disk of the light beam. Besides, in order to cause the light spot to follow up the movement of the information track, the tracking servo control is employed by using a tracking error signal produced by the reflected light from the optical disk of the light beam.
As a tracking servo controlling method, there are a so-called three-beam method and a so-called push-pull method. When a light spot crosses an information track, the tracking error signal S.sub.TE obtained by each of these methods has a waveform as shown in FIG. 41, for example. In the tracking servo control, by using the tracking error signal S.sub.TE shown in FIG. 41, a displacement (deviation) of the light spot from the information track is detected, and on the basis of the detected displacement, the objective lens on the optical path of the light beam is moved in a direction perpendicular to the information track by an actuator or the like, to thereby control the position of the light spot.
In the above mentioned methods, however, when the track pitch is narrowed, a sufficient tracking error signal cannot be obtained.
That is, supposing the wavelength of the light beam to be .lambda., and the numerical aperture of the objective lens to be NA, the optical cut-off wavelength is expressed by an expression of .lambda./(2.times.NA), and when the track pitch is not larger than this optical cut-off wavelength, a sufficient tracking error signal is not obtained.
Further, even if the track pitch is larger than the optical cut-off wavelength, if the light beam is not focused at the optimum focal length and the size of the light spot is large enough to receive the effect of crosstalk from the adjacent track thereto, a sufficient tracking error signal may not be obtained.
That is, when reproducing an optical disk, if the light spot formed on the optical disk is lager than the condensing or focusing limit of (0.85.times..lambda.)/NA due to the effect of aberration and becomes close to the first dark ring of (1.22.times..lambda.)/NA, the light spot is likely to receive the effect from the track adjacent to the presently reproduced track.
In this manner, in case that the track pitch is not larger than the optical cut-off wavelength or the light spot is not focused at the optimum focal distance, even if the central position of the light beam is at a mirror surface position (hereinafter it is referred to as a "mirror") between two tracks adjacent to each other (on each of which a "pit" is arranged), the reproduction signal is affected by both of these tracks, and the change of the reproduction signal associated with the movement of the light beam in a radial direction is slow. If the arrangement of the pit and the mirror in the radial direction is inverted, the direction of this change is also inverted.
The pits and the mirrors are recorded on the optical disk such that the incidence probabilities of the pits and the mirrors may be equal to each other. By combination of the pits and the mirrors, waveforms different in the changing direction of the reproduction signal respectively appear symmetrically. Further, since the reproduction signal is detected through a low-pass filter, these symmetrical waveforms are integrated and averaged, and hence the tracking error signal cannot be detected.
Therefore, according to the above mentioned tracking error controlling method, there are such drawbacks that the tracking pitch cannot be narrowed and the information recording medium cannot be enhanced in density.